This invention generally relates to a cover for a body of water (e.g., lagoons, ponds and the like), and particularly relates to a cover for anaerobic sewage lagoons (particularly agricultural sewage lagoons) to reduce odors and emissions of ammonia, hydrogen sulfide and/or volatile organic compounds from such lagoons into the environment.
Agricultural sewage lagoons may extend over large areas (e.g., several hectares) and often produce, for example, various odor, ammonia (NH3), hydrogen sulfide (H2S) and/or volatile organic compound (VOC) emission problems. Numerous attempts have been made to reduce such odor and gas emissions.
Clanton et al. (in C. J. Clanton, D. R. Schmidt, L. D. Jacobson, R. E. Nicolai, P. R. Goodrich, K. A. Janni, xe2x80x9cSwine Manure Storage Covers for Odor Control,xe2x80x9d Applied Engineering in Agriculture, Vol. 15(5), pp. 567-72 (1999)) disclose six alternative cover treatments for reducing odors from agricultural sewage lagoons: a straw mat, a vegetable oil mat, a mat formed from a combination of vegetable oil and straw, a clay ball mat, a PVC/rubber membrane (PERMALON, Reef Industries, Inc., Houston, Tex. U.S.A.) composed of high density polyethylene and supported on STYROFOAM floats sewn into pockets at the periphery of the membrane and a self-floating cover consisting of a paper thin (0.3 mm) xe2x80x9cgeotextile membranexe2x80x9d (TYPAR 3201C, DuPont Geotextiles North America, Nashville, Tenn., U.S.A.) composed of thermally bonded, continuous polypropylene filaments. Clanton et al. report that all six alternatives reduced odors and hydrogen sulfide emitted from swine sewage tanks. None of these alternatives, however, are entirely satisfactory. The straw mat, for example, deteriorates over time and also sinks during periods of heavy rain. The vegetable oil mat, on the other hand, produces a xe2x80x9cdistinctively offensive non-swine odorxe2x80x9d and is not recommended as a cover. The clay ball mat did not perform as well as expected and is the least effective alternative for reducing emissions. Clanton et al. speculate that these results may stem from differences in clay ball materials, types and sizes. Although Clanton et al. report that the PVC/rubber membrane consistently reduces both odors and hydrogen sulfide, such a cover is disadvantageous because it is not self-floating (i.e., it requires STYROFOAM supports to stay afloat). Clanton et al. report that odor reduction by the xe2x80x9cgeotextile membranexe2x80x9d is minimal. Nevertheless, Clanton et al. speculate that such a mat would be more effective over time if a thick biofilm is allowed to grow on the upper surface of the membrane to an extent sufficient to produce a seal to prevent odors and gases from escaping from the lagoon into the environment. Clanton et al., however, fail to recognize that such a self-sealed cover tends to collect rain water and will ultimately sink during periods of heavy rain. This disadvantage also would tend to occur with the PVC/rubber membrane cover. In addition, such a self-sealed geotextile cover and the PVC/rubber membrane cover tend to only delay at least some of the offensive odors and gases from escaping from the lagoon; these gases would simply tend to collect between the cover and the lagoon surface, thereby causing gas bubbles under the cover and ultimate emission of the offensive odors and gases, for example, at the edges of the cover. In addition, it is difficult to remove dead biomass from the geotextile and PVC/rubber covers.
Betsy Freese (in B. Freese, xe2x80x9cManure Storage Covers,xe2x80x9d Successful Farming, September, 1997) also summarizes a variety of alternative covers for reducing odors from agricultural sewage lagoons: a straw mat, a mat made from cornstalks, the PERMALON floating plastic cover (Reef Industries, Inc., Houston, Tex., U.S.A.) composed of high density polyethylene, a cover made from peat moss, a floating cover made from thin open celled polyurethane foam and a cover made from leka rock (i.e., a porous material made from lava rock and coated with an impervious material baked on the surface). For the reasons discussed above, the straw mat is not entirely satisfactory. The mats made from cornstalks and peat moss suffer from the same disadvantages as the straw mat and the peat moss is even less desirable due to its costs compared to straw and cornstalks. The PERMALON floating plastic cover and the thin open celled foam cover suffer from problems analogous to those discussed above with respect to the PVC/rubber and geotextile membrane covers of Clanton et al. Although the leka rock was found to reduce offensive gas and odor emissions and resist deterioration, it is uneconomical due to its high costs.
U.S. Pat. No. 5,861,095 and U.S. Pat. No. 5,400,549 disclose covering sewage lagoons with a series of rectangular insulated panels which are connected to each other through a grommet and cable system. Such an insulated cover is reported to reduce the odors emitted from a lagoon by (a) blocking sunlight to the surface of the lagoon, thereby decreasing the amount of algae growth on the lagoon surface; and (b) providing insulation from ambient air temperatures (e.g., freezing temperatures) to maintain a greater lagoon temperature year round, thereby increasing the activity of the bacteria in the lagoon for greater biodegradation of organic material in the lagoon. Such a cover reportedly remains buoyant even after rainfall because the rain water is able to drain through the gaps between the panels. Such gaps between the panels, however, also allow odors and harmful gases to be freely emitted from the lagoon into the atmosphere. In addition, the panels are bulky and therefore cumbersome to handle and difficult to discard.
Other covers are reported by J. Ronald Miner et al. in xe2x80x9cA Floating Permeable Blanket to Prevent Odor Escape,xe2x80x9d Proc. Int. Livestock Odor Conf., (Iowa State University, Ames Iowa, 1995), pp. 28xe2x80x9434. These covers included (1) a cover comprising a single layer of open celled polyurethane foam supported by a layer of closed cell polyethylene foam; (2) a cover comprising 2 layers of open celled polyurethane foam supported by a layer of closed cell polyethylene foam; (3) a cover comprising a layer of closed cell polyethylene foam; and (4) covers comprising 2 layers of open celled polyurethane foam supported by a layer of closed cell polyethylene foam and having varying amounts of zeolite between the layers of polyurethane foam. In all the covers, the closed cell polyethylene was perforated with 3 to 4 mm holes drilled in a 2 cm grid pattern.
Despite the foregoing, a need continues to exist for an improved cover which is effective in reducing odor and offensive gas emissions from sewage lagoons. There is particularly a need for a cover having an improved high-surface-area portion suitable for promoting the growth of aerobic biomass into which offensive gases from the lagoon can permeate and be destroyed through oxidation.
This invention provides for an improved cover for a body of water (e.g., a pond, swimming pool, sewage lagoon, fisheries and other aquacultural bodies of water, etc.). In particularly preferred embodiments, this invention provides for a self-floating cover which: (a) may be used to reduce odor, ammonia, hydrogen sulfide and volatile organic compound emissions from a sewage lagoon; (b) has a structure suitable for growing a layer of aerobic biomass (in particular, aerobic bacteria and/or aerobic algae) into which offensive gases from a sewage lagoon may permeate to be oxidized and thereby destroyed; (c) has sufficient buoyancy such that, even with maximal biomass growth on and within the cover, a significant portion of the cover and biomass remains above the surface of a sewage lagoon to enable the maintenance of a zone which comprises aerobic biomass into which gases from the lagoon may permeate to be oxidized and destroyed; (d) is permeable to rain water even when maximal biomass growth exists on and within the cover; and/or (e) is structured so that dead biomass on and within the cover can be washed off the cover by rain water or other source of water to better enable the continuous regeneration of living biomass on and within the cover.
Briefly, therefore, the present invention is generally directed to a self-floating cover system for a body of water.
One embodiment is directed to a self-floating cover system for reducing offensive gases and/or odors emitted from a body of water (e.g., a sewage lagoon). The cover system comprises at least one cover unit which is sized and shaped to cover at least a portion of the body of water. The cover unit is flexible and has an upper surface formed by a filamentary material (e.g., a synthetic filamentary material) capable of supporting growth of aerobic biomass (e.g., aerobic bacteria and/or aerobic algae) when the cover system is placed onto the surface of the body of water. The buoyancy of the cover unit is greater than the maximum weight of biomass that accumulates on and within the cover unit when the cover system is placed onto the surface of the body of water such that at least a portion of the upper surface of the cover unit remains above the surface of the body of water.
Another embodiment is directed to a self-floating cover system for a body of water. The cover system comprises at least one cover unit sized and shaped to cover at least a portion of the body of water. The cover unit is flexible and has a surface which is formed by a filamentary material (e.g., a synthetic filamentary material). This filamentary material is at least about 0.3 cm thick, has a unit weight of from about 2 to about 15 ounces/yd2 and is capable of supporting growth of aerobic biomass thereon when the cover system is placed onto the surface of the body of water.
A further embodiment is directed to a self-floating cover system for reducing offensive gases and/or odors emitted from a sewage lagoon. This cover system comprises at least one cover unit sized and shaped to cover at least a portion of the sewage lagoon. The cover unit is flexible and has a bottom surface for resting above or below the surface of the sewage lagoon and an upper surface formed by a synthetic material having a surface area which is greater than the surface area of the material forming the bottom surface. The synthetic material forming the upper surface is capable of supporting the growth of aerobic biomass when the cover system is placed onto the surface of the sewage lagoon.
This invention is also directed to a method for using these covers on a body of water, and particularly on a sewage lagoon to reduce the emission of offensive odors and/or gases.
This invention is further directed to sewage treatment systems comprising the covers of this invention on the surface of a sewage lagoon.
Other objects and features of this invention will be in part apparent and in part pointed out hereinafter.